<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width,initial-scale=1,minimum-scale=1,maximum-scale=1,user-scalable=no" />
  <title>Document</title>
  <style>
    *{
      margin: 0;
      padding: 0;
    }
    html,body{
      height: 100%;
      /* font-size: xxxxvw; */
    }
    body{
      height: 100%;
      display: flex;
      flex-direction: column;
    }
    header{
      height:3.5rem;
      background-color: red;
    }
    footer{
      height: 3.75rem;
      background-color: red;
    }
    main{
      flex: 1;
      overflow-y: auto;
      background-color: aqua;
    }
  </style>
</head>
<body>
  <header>
    <div></div>
  </header>
  <main></main>
  <footer></footer>
</body>
</html>